Before the COBE detection, there had been about 25 years of quoted
upper limits to CMB anisotropy. There was much talk in the literature about
how new paradigms would be required if COBE returned yet stronger
upper limits. But in fact the detection was just at the right level for
gravity alone to have grown the structure from amplitudes of
~ 10-5 at z ~ 1000 to the non-linear structures we see
today. This is easy to arrange for models in which the Universe is
dominated by non-baryonic dark matter, and with adiabatic perturbations.
Thus, probably the most important thing to come out of the COBE
anisotropy measurements (apart from the general good news that we are
on the right track!) is the realization that

This `fact', arising from the CMB,
has added to the Big Bang paradigm, so that the picture is
of a hot expanding Universe, which at early times contained
small amplitude density perturbations. The obvious next question is where
those perturbations came from - an issue we shall return to in a minute.

Figure 3. The result of binning the data in
the previous figure.
More precisely, what was done was to split the multipoles into
16 bins between l = 2 and l = 1000, and to weight each
experiment
by the fraction of the window in each bin. The precise height of the
peak depends to some extent on the choice of bins, on details of the
window functions used, and on the weights given to individual experiments.
The points here are not un-correlated, but provide a reasonable visual
impression of the current data - more sophisticated treatments
(e.g. [32]
give similar results). The solid line is a
-dominated CDM model, with
parameters which
are consistent with most current cosmological constraints.

Meanwhile, there are a few other things that the current suite of CMB
measurements tells us. First of all, it is pleasing that the
approximate scale of the peak (apparent in the binned plot,
Figure 3)
is just where it is theoretically predicted
in simple models. This acoustic peak corresponds to the length scale which a
sound wave can travel at the time of recombination, projected onto
the sky, and was contained in papers at least as early as Doroshkevich,
Sunyaev & Zel'dovich (1978)
[31].
The position in angle is also
a good test for the geometry of the Universe, since it comes from the
projection of a fixed physical scale onto the sky.

We know (from the lack of complete absorption
shortwards of the Lyman edge in distant quasars) that most of the material
in the nearby Universe is ionized, out to redshifts z > 5.
Whether the Universe reionized at z ~ 10 or z ~ 1000 is,
however, not obvious. But, very early reionization would lead to the erasing
of the small-scale CMB anisotropies, which patently has not
occurred. Hence we can infer that